The mechanisms of action of two enzymes of fatty acid metabolism will be studied from a stereochemical point of view. Beta-Hydroxydecanoylthioester dehydrase ("dehydrase") is the enzyme at the branch point in the biosynthesis of saturated and unsaturated fatty acids by anaerobic bacteria. Dehydrase's normal function is to catalyze the dehydration of 3-hydroxydecanoyl acyl carrier protein to the trans-2-decenoyl thio-ester followed by isomerization to the cis-3-decenoyl compound. Dehydrase is irreversibly inhibited by 3-decynoic acid derivatives, which are isomerized to 2,3-decadienoates. The latter compounds inactivate the enzyme by alkylation of an active site histidine. A second enzyme, from hog liver, catalyzes the identical acetylene-allene isomerization reversibly and without inactivation by substrate. The stereochemical modes of action of these two enzymes will be studied. These studies will involve (a) synthesis of substrates labeled stereoselectively with deuterium, (b) chemical degradation and deuterium NMR analysis of chirally-labeled products, and (c) determination of the absolute configuration of the allene produced by the hog liver enzyme. Suprafacila and antarafacial modes of action detected in this way are indicative of one- and two-base carbanion mechanisms, respectively.